Whereabouts are they located?
well this was on an omega, but as your model astra was around the time the omega was i assume some similarity...may be wrong though.
They were at the back of the engine, middle left iirc, but look at your haynes manual under sensors or what have you and make sure they are connected properly for both TC and ABS and place some electrical tape on if necessary.
It went into vauxhall several times and they could not sort this out btw.

